This invention is directed to a method and apparatus for molding of plastic bottles. More particularly, it is directed to the hot-blow or one-step process for the manufacture of molecularly oriented plastic bottles having a high degree of blow mold utilization and overall efficiency.
In recent years substantial effort has been directed to the formation of plastic bottles as a replacement, or partial replacement of glass bottles. This effort has built on knowledge that plastic such as polyethylene terephthalate (PET) when molecularly stretched is tougher but lighter than glass and the glass art which describes both methods and apparatus for stretching and blowing glass in both the longitudinal and transverse directions.
According to the prior art, molecularly oriented containers such as plastic bottles have utilized either a reheat or two-stage process and system, or a hot-blow or one-stage process and system. In the reheat or two-stage process, parisons are first injection molded in a parison mold, cooled to room temperature, and stored for subsequent blowing into the finished bottle in a blow mold. At time of blowing, the parisons are reheated and brought to the blowing temperature of the plastic prior to blowing. In the hot-blow or one-stage process and system, the parison is injection molded in a parison mold and substantially immediately after formation is transferred at the blowing temperature to a blow mold where the parison is blown into the finished bottle.
Both of the above-noted systems have advantages and disadvantages. In the reheat process and system, efficient and effective use can be made of both the parison-forming and blowing stations which need not be integrated. However, substantial thermal energy is lost during the total operation in that the parison after formation is cooled down during storage and then reheated at time of blowing. Moreover, the parisons from storage must be fed to the blowing station, duplicating handling. The hot-blow process and system eliminates heat loss and duplicate handling. However, the advantages of the hot-blow process and system are mitigated in that conventionally, in order to have proper temperature conditioning of the parison at the blowing stage, for each parison station there is a corresponding blowing station. Since the time required for the parison forming stage, including temperature conditioning, is substantially longer than the blowing stage, i.e., approximately 20 seconds versus 5 seconds for polyethylene terephthalate (PET), inefficient use is made of the blowing stations.